1. Field of the Invention
This invention relates to the preparation of polyorganosiloxanes. More particularly, this invention relates to a method and certain novel catalyst compositions for preparing polyorganosiloxanes from one or more organosiloxanes containing silicon-bonded hydroxyl groups.
2. Description of the Prior Art
Polyorganosiloxanes are well known materials, and range in consistency from liquids to resinous or elastomeric solids at room temperature. The physical state of a given polyorganosiloxane is a function of its molecular weight and the type and number of hydrocarbon groups bonded to silicon.
Polyorganosiloxanes have been prepared by a variety of methods. In accordance with one such method, one or more silanes or siloxanes containing silicon-bonded hydroxyl groups are reacted in the presence of suitable condensation catalysts. U.S. Pat. No. 3,308,203, which issued to Metevia and Polmanteer on Mar. 7, 1967, discloses several classes of compounds that catalyze the condensation of silicon-bonded hydroxyl groups. Among the classes of catalysts disclosed are alkali metal hydroxides, organosilicon salts of alkali metal hydroxides, primary, secondary and tertiary amines, aromatic sulfonic acids, fluoroalkanesulfonic acids, and salts obtained by the reaction of "basic amino compounds" with either phosphoric acid or carboxylic acids. The aforementioned patent teaches that these catalysts do not cause excessive siloxane bond rearrangement, and are therefore useful for preparing organosiloxane block copolymers.
Applicant has found that some of the preferred catalysts exemplified in the aforementioned patent either will not produce polyorganosiloxanes of sufficiently high molecular weight to yield elastomers and resins having commercially useful physical properties, or the catalysts must be used in such large concentrations that they adversely affect the physical or chemical properties of the final cured polyorganosiloxane.
An additional problem associated with using many prior art catalysts is that in addition to accelerating the reaction between two silicon-bonded hydroxyl groups to form .dbd.SiOSi.dbd. bonds, at the required use levels the catalysts may accelerate a hydrolysis of previously formed silicon-oxygen bonds. This can result in the formation of cyclic organosiloxane oligomers and a reduction in the average molecular weight of the polymer. The cyclic organosiloxanes typically have relatively low boiling points and are readily distilled from the reaction mixture together with the liquid hydrocarbon often employed as a reaction medium. The formation of cyclic organosiloxanes place an upper limit on the average molecular weight that can be achieved under a given set of conditions. In addition, the rearrangement and equilibration of siloxane bonds could destroy the alternating sequences of repeating units that characterize the structure of block copolymers.
An objective of this invention is to provide a method and novel catalyst compositions for preparing high molecular weight polyorganosiloxanes, including block copolymers, by a condensation reaction between one or more hydroxyl-containing polyorganosiloxanes. An additional objective is to provide novel catalysts for the reaction between silicon-bonded hydroxyl groups that are effective at concentration levels which do not adversely affect electrical and other properties of the resultant polymer.